Did you permanently remove a lot of mail lately? Did you wait for delted items retention to expire? has online maintenance and an onling defrag completed since DIR expired? How much whitespace is in the database in question (event 1221)?
Unless you permanently deleted a large amount of mail, it's not worth doing the offline defrag. If you're running Enterprise or have another exchange server in your environment, you'd be far better off doing move mailbox. You get all the upside (yes, it preserves SIS) and the user impact is minimal. After you move the mailboxes, simply dismount and delete the old database.